Doushantuo Formation In South China Research Articles

Fossil microbial communities in Neoproterozoic interglacial rocks, Maikhanuul Formation, Zavkhan basin, Western Mongolia

Glaciogenic deposits are widely distributed in the Neoproterozoic successions around the world, but only few of them are associated with occurrences of authentic macroscopic fossils. Problematical circular remains recovered from intertillite beds of the Neoproterozoic Maikhanuul Formation, Zavkhan basin, Western Mongolia may be a new record of biogenic structures in ancient glacial rocks. The Maikhanuul tillites were deposited between 732 and 777Ma (the underlying volcanic rocks of the Neoproterozoic Dzabkhan Formation) and 632±14Ma (the overlying limestones of the Ediacaran – Nemakit – Daldynian Tsagaanolom Formation). The assemblage of microfossils from the Tsagaanolom Formation is comparable to the assemblage known from the 635–551Ma Doushantuo Formation of South China. The Maikhanuul glacial event may therefore be correlated with the 635Ma Nantuo glaciation. Morphological, taphonomic, biometric and geochemical observations have allowed us to interpret the problematical circular remains from the Maikhanuul tillites as microbial communities preserved in situ on the floor of an ancient glacial basin. Many discoidal fossils from Precambrian strata have been compared with extant microbial colonies that have a similar external morphology. However, ultrastructure of the hypothetical Precambrian microbial communities, to the best of our knowledge, has not previously been reported. Scanning electron microscopy of the circular remains from the Maikhanuul tillites revealed rod-shaped, filamentous and star-shaped bacteriomorph structures. Some microremains and especially the star-shaped particles are considerably enriched in iron (up to 50%) that may be attributed to bacterial activity. Furthermore, the star-shaped microremains can be compared with modern star-shaped bacteria known from freshwater and terrestrial environments and with aggregates of biogenic nanohematite. Although the presence of microbial communities could not be used for global correlation, it does provide information on prokaryotic diversity and ecology as well as on palaeoenvironmental conditions.

Ediacaran Acanthomorphic Acritarchs and Other Microfossils from Chert Nodules of the Upper Doushantuo Formation in the Yangtze Gorges Area, South China

Silicified microfossils preserved in chert nodules of the Doushantuo Formation in the Yangtze Gorges area of South China have great potential to improve the biostratigraphic subdivision and correlation of the Ediacaran System. This potential can be realized only if solid taxonomy is available. However, a systematic treatment of these microfossils (particularly acanthomorphic acritarchs) is lacking, greatly limiting their biostratigraphic potential. This paper presents the systematic paleontology of silicified microfossils from upper Doushantuo Formation (Member III) chert nodules at three sections in the Yangtze Gorges area. More than 90 species of microfossils are described, including 66 named taxa of acanthomorphs, seven named taxa of sphaeromorphs, 12 taxa of cyanobacterial filaments and coccoids, four taxa of algal thalli, and two species of tubular microfossils. Several acritarch species, includingAppendisphaera clavan. sp.,Mengeosphaera grandispinan. sp.,M. stegosauriformisn. sp.,Leiosphaeridia, and possiblySinosphaera rupina, are shown to be multicellular organisms, consistent with the proposition that some Ediacaran acritarchs may be diapause eggs of early animals. This study supports the view that theTianzhushania spinosaacanthomorph biozone is unique to the lower Doushantuo Formation in South China (and perhaps its equivalent in northern India) and that Ediacaran acanthomorph assemblages from Australia, Siberia, and East European Platform are younger than theTianzhushania spinosabiozone. It is proposed that the first occurrence ofHocosphaeridium anozos, a species with easily recognizable morphology and wide taphonomic and geographic distributions, be used to define the second Doushantuo acanthomorph biozone succeeding theTianzhushania spinosabiozone. New taxa described in this paper include three new genera (Bispinosphaeran. gen.;Yushengian. gen.; andGranitunican. gen.) and 40 new species:Appendisphaera?brevispinan. sp.,A. clavan. sp.,A.?hemisphaerican. sp.,A. longispinan. sp.,A. setosan. sp.,Bispinosphaera peregrinan. gen. n. sp.,Crinita paucispinosan. sp.,Ericiasphaera densispinan. sp.,Hocosphaeridium dilatatumn. sp.,Knollisphaeridium denticulatumn. sp.,K. longilatumn. sp.,K. obtusumn. sp.,K. parvumn. sp.,Mengeosphaera angustan. sp.,M. bellulan. sp.,M.cf.bellulan. sp.,M. constrictan. sp.,M.?cuspidatan. sp.,M.?gracilisn. sp.,M. grandispinan. sp.,M. latibasisn. sp.,M. miniman. sp.,M. spicatan. sp.,M. spinulan. sp.,M. stegosauriformisn. sp.,M. triangularisn. sp.,M. uniformisn. sp.,Sinosphaera asteriformisn. sp.,Tanarium acusn. sp.,T. elegansn. sp.,T. longitubularen. sp.,T.?minimumn. sp.,T. obesumn. sp.,T. variumn. sp.,Urasphaera fungiformisn. sp.,U. nuptan. sp.,Yushengia ramispinan. gen. n. sp.,Granitunica mcfaddeniaen. gen. n. sp.,Osculosphaera arcelliformisn. sp., andO. membraniferan. sp.

The nature and origin of nucleus‐like intracellular inclusions in Paleoproterozoic eukaryote microfossils

The well-known debate on the nature and origin of intracellular inclusions (ICIs) in silicified microfossils from the early Neoproterozoic Bitter Springs Formation has recently been revived by reports of possible fossilized nuclei in phosphatized animal embryo-like fossils from the Ediacaran Doushantuo Formation of South China. The revisitation of this discussion prompted a critical and comprehensive investigation of ICIs in some of the oldest indisputable eukaryote microfossils-the ornamented acritarchs Dictyosphaera delicata and Shuiyousphaeridium macroreticulatum from the Paleoproterozoic Ruyang Group of North China-using a suite of characterization approaches: scanning electron microscopy (SEM), transmission electron microscopy (TEM), and focused ion beam scanning electron microscopy (FIB-SEM). Although the Ruyang acritarchs must have had nuclei when alive, our data suggest that their ICIs represent neither fossilized nuclei nor taphonomically condensed cytoplasm. We instead propose that these ICIs likely represent biologically contracted and consolidated eukaryotic protoplasts (the combination of the nucleus, surrounding cytoplasm, and plasma membrane). As opposed to degradational contraction of prokaryotic cells within a mucoidal sheath-a model proposed to explain the Bitter Springs ICIs-our model implies that protoplast condensation in the Ruyang acritarchs was an in vivo biologically programmed response to adverse conditions in preparation for encystment. While the discovery of bona fide nuclei in Paleoproterozoic acritarchs would be a substantial landmark in our understanding of eukaryote evolution, the various processes (such as degradational and biological condensation of protoplasts) capable of producing nuclei-mimicking structures require that interpretation of ICIs as fossilized nuclei be based on comprehensive investigations.

Hydrothermal alteration of the Ediacaran Doushantuo Formation in the Yangtze Gorges area (South China)

The geochemical and fossil record preserved in the Ediacaran age (635–551Ma) Doushantuo Formation of South China has been extensively examined to explore the impact of changing climate and the oxidation state of the oceans on the development and distribution of early multicellular life. In the Yangtze Gorges area, this formation shows many of the geochemical trends and features thought to typify global ocean chemistry in the Ediacaran Period, but there are indications that post-sedimentary processes modified these signals. This study of clay minerals and organic matter builds a more detailed picture of the type and degree of post-sedimentary alteration at different stratigraphic levels of the formation and focuses on how this alteration influenced stable carbon and oxygen isotope records.In the cratonward Jiulongwan and Huajipo sections of the Doushantuo Formation, its lower part (Members 1 and 2) consists largely of dolomitic shale, rich in authigenic saponite that crystallized in an alkaline sedimentary basin. Saponite has been altered to chlorite via corrensite across tens of meters of strata in lower Member 2, with increased alteration downward toward the cap dolostone. The greater chloritization is accompanied by lower δ18O and higher δD values of trioctahedral clays. This pattern of alteration of trioctahedral clays is likely due to hydrothermal fluid activity in the underlying, relatively permeable Nantuo Formation and cap dolostone. A concomitant increase of solid bitumen reflectance toward the base of the formation supports this idea. In the uppermost part of the formation in the Yangtze Gorges area (Member 4), a typical open water marine dolomitic shale rich in illite and organic matter, increases in the methylphenanthrenes ratio index and solid bitumen reflectance correlate with decrease of the bulk rock K/Al ratio upward, providing evidence for hot fluid migration above the nearly impermeable shale.Clay from the upper part of the formation is enriched in 18O, but not in D, relative to clay from the lower parts, indicating progressive 18O-enrichment of hydrothermal fluids that percolated upward and laterally through permeable 18O-rich carbonates. A maximum hydrothermal-alteration temperature of ∼200°C is estimated from a calibration curve for illitization during burial diagenesis, but given that the hydrothermal activity probably occurred in short pulses, the temperature could have been much higher. K–Ar ages are consistent across different size fractions of fine illite from Member 4 shale (∼430Ma) and from a K-bentonite bed near the base of Member 2 in the Jiuqunao section (∼325Ma), ∼25km from Jiulongwan and Huajipo. These age values show that the diagenetic illite of the Doushantuo Formation is a product of either deep burial diagenesis overprinted by spatially limited hydrothermal activity or of two localized hydrothermal events.Patterns of carbonate 13C and 18O depletion in the basal Doushantuo Formation are similar to chloritization trends and 18O variation in diagenetic clay minerals. Given independent evidence for 13C depletion of hydrothermal fluids, these trends indicate carbonate–fluid isotope exchange commensurate with the degree of post-sedimentary alteration, supporting a model of lithologically controlled differential diagenesis induced by hydrothermal fluids as the main control on C and O isotope variability in this stratigraphic interval. This model could potentially explain other notable δ13C excursions higher up in Member 3.

Ocean oxygenation in the wake of the Marinoan glaciation

Metazoans are likely to have their roots in the Cryogenian period, but there is a marked increase in the appearance of novel animal and algae fossils shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago. It has been suggested that an oxygenation event in the wake of the severe Marinoan glaciation was the driving factor behind this early diversification of metazoans and the shift in ecosystem complexity. But there is little evidence for an increase in oceanic or atmospheric oxygen following the Marinoan glaciation, or for a direct link between early animal evolution and redox conditions in general. Models linking trends in early biological evolution to shifts in Earth system processes thus remain controversial. Here we report geochemical data from early Ediacaran organic-rich black shales (∼635-630 million years old) of the basal Doushantuo Formation in South China. High enrichments of molybdenum and vanadium and low pyrite sulphur isotope values (Δ(34)S values ≥65 per mil) in these shales record expansion of the oceanic inventory of redox-sensitive metals and the growth of the marine sulphate reservoir in response to a widely oxygenated ocean. The data provide evidence for an early Ediacaran oxygenation event, which pre-dates the previous estimates for post-Marinoan oxygenation by more than 50 million years. Our findings seem to support a link between the most severe glaciations in Earth's history, the oxygenation of the Earth's surface environments, and the earliest diversification of animals.

A merciful death for the “earliest bilaterian,” Vernanimalcula

Fossils described as Vernanimalcula guizhouena, from the nearly 600 million-year-old Doushantuo Formation in South China, have been interpreted as the remains of bilaterian animals. As such they would represent the oldest putative record of bilaterian animals in Earth history, and they have been invoked in debate over this formative episode of early animal evolution. However, this interpretation is fallacious. We review the evidential basis of the biological interpretation of Vernanimalcula, concluding that the structures key to animal identity are effects of mineralization that do not represent biological tissues, and, furthermore, that it is not possible to derive its anatomical reconstruction on the basis of the available evidence. There is no evidential basis for interpreting Vernanimalcula as an animal, let alone a bilaterian. The conclusions of evolutionary studies that have relied upon the bilaterian interpretation of Vernanimalcula must be called into question.

Greigite from carbonate concretions of the Ediacaran Doushantuo Formation in South China and its environmental implications

Greigite (Fe3S4) is a ferrimagnetic iron sulfide that commonly forms as a precursor of pyrite in anoxic environments where the supply of reactive Fe outpaces that of sulfide (H2S and HS−). Because of its metastability and sensitivity to redox changes during burial, greigite has been rarely documented in rocks older than the Cretaceous. Here we report well-preserved greigite in carbonate concretions of the upper Doushantuo Formation (ca. 551Ma) in the Yangtze Gorge area, South China. Greigite in the carbonate concretions coexists with anhedral and framboidal pyrite, and is distributed in clay-rich carbonates with card-house microtextures and dolomitic spherical structures indicative of early diagenetic formation during shallow burial. Preservation of greigite in carbonate concretions of the upper Doushantuo Formation implies that these concretions maintained a closed micro-system since their formation and that they provide information about the ancient depositional and early diagenetic environments.

Rare earth elements and carbon isotope geochemistry of the Doushantuo Formation in South China: Implication for middle Ediacaran shallow marine redox conditions

The middle Ediacaran Shuram excursion, the largest negative δ13Ccarb excursion in Earth history, has been interpreted as indirect evidence for episodic oxidation and remineralization of deep ocean DOC (dissolved organic carbon). It has been hypothesized that such oxidation event may have occurred when anoxic DOC-laden deep water was brought to shallow shelves during oceanic upwelling, which is expected to cause localized anoxia in shallow environments. To test this prediction, we systematically analyzed rare earth elements (REE) and δ13Ccarb of the upper Doushantuo Formation carbonates in the Yangtze Gorges area of South China, which were deposited in an inner shelf environment and record a large negative δ13Ccarb excursion correlated to the Shuram event. The REE data show a significant positive shift in Ce/Ce* values, synchronous with a pronounced negative δ13Ccarb shift. This positive Ce/Ce* shift is interpreted to represent an oceanic anoxia event in shallow shelf environments, which may have been caused by the upwelling or impingement of oxygen-depleted and 12C-enriched deep water onto shelves. This anoxia event coincides with a sharp decline in the abundance and diversity of Ediacaran acanthomorphic acritarchs, raising the possibility that these two geobiological events may be causally related.

Diverse small spinose acritarchs from the Ediacaran Doushantuo Formation, South China

The Ediacaran Doushantuo Formation in South China is underlain by the Cryogenian Nantuo Formation (glacial rocks) and overlain by the late Ediacaran Dengying Formation. It is characterized by well-preserved, large (normally >100 μm in size) spinose acritarchs (LSAs), which have been shown to be probably the only useful biostratigraphic tool for the global correlation of the early- and middle-Ediacaran. Acritarchs are organic microfossils normally known as single-celled eukaryotic organisms (protists). Although recent research suggests that some large spinose acritarchs may represent diapause egg cysts of metazoans, the biological affinities of the Ediacaran spinose acritarchs, especially for those displaying remarkable size ranges, are still debatable. Recently, smaller specimens of the Ediacaran spinose acritarchs have been found in cherts and phosphorites of the Doushantuo Formation in South China. Many described Ediacaran spinose acritarch taxa display large size variation (from tens to hundreds of microns in vesicle diameter), but some taxa only have smaller (<70 μm) specimens. The morphological comparison with Paleozoic counterparts indicates that some Ediacaran spinose acritarchs may have phylogenetic affinity to eukaryotic algae. More evidence, including wall ultra-structure, geochemical analysis and comparison with modern analogs, is needed to understand the biological affinity of the Ediacaran spinose acritarchs. The remarkable radiation of planktonic protists, characterized by abundant, diverse spinose acritarchs, occurred as early as in the late Neoproterozoic, i.e., 40–60 million years earlier than previously thought.

Carbon isotope chemostratigraphy and sedimentary facies evolution of the Ediacaran Doushantuo Formation in western Hubei, South China

The Doushantuo Formation of South China, consisting of intercalated siliciclastic and carbonate rocks which exhibit considerable variation in thickness and sedimentary facies, is one of the key sedimentary units in the world for understanding Ediacaran geobiological processes. However, stratigraphic correlation and sedimentary facies evolution of the Doushantuo Formation remain controversial. Sedimentary facies analysis of the well-known sections of the Yangtze Gorges area and new sections in northern Yichang, western Hubei indicate that the Doushantuo succession in the Yangtze Gorges area was deposited in an offshore intra-shelf basin within the Yangtze platform. In contrast, the Doushantuo Formation in the northern Yichang area which is characterized by one or a few phosphate-rich horizons represents deposition on the subtidal or intertidal shelf, and so is similar to the succession in the Weng’an area (central Guizhou Province). The carbon isotope profiles of the different sections exhibit significant variation, which is interpreted to result from facies differences. Sequence and high-resolution carbon isotope stratigraphy have clarified problematic correlations and the temporal relationship between fossil assemblages of the Doushantuo Formation on the Yangtze platform. The results confirm that a prominent negative δ13Ccarb excursion (BAINCE, nadir down to −5‰) in the middle portion of the second sequence of the Doushantuo Formation is a widely traceable event. BAINCE together with three other δ13Ccarb excursions, namely CANCE at the base, WANCE near the first sequence boundary near the middle, and DOUNCE at the top of the Doushantuo Formation, constitute a complete carbon isotope profile for the Doushantuo Formation that reflects contemporaneous seawater and so serves as a reference chemostratigraphic profile for global stratigraphic correlation and for evaluating the chemical evolution of seawater during the Ediacaran Period.

Evolution of animal multicellularity stimulated by dissolved organic carbon in early Ediacaran ocean: DOXAM hypothesis

AbstractOxygenation of the ocean is presumed to be an important factor stimulating the evolution of multicellular animals. The appearance of the Ediacaran‐type biota (ca 575 Ma) was assigned to the aftermath of the Gaskiers glaciation (ca 580 Ma), when substantial oceanic oxygenation is believed to have started. However, several lines of evidence reveal that at least sponges evolved before this oxygenation. For understanding the first stage of animal evolution, we propose the hypothesis that Dissolved Organic Carbon (DOC) Stimulated the evolution for Animal Multicellularity (DOXAM). Recent geochemical studies of the Ediacaran sedimentary sequences have indicated that a substantial DOC mass was developed in the stratified ocean after the Marinoan glaciation (655–635 Ma), and this was supported by the inorganic and organic carbon isotope profiles of the Doushantuo Formation in South China. The DOC mass was an oxygen consumer in the water column; however, it could have provided a food source for filter‐feeding animals such as sponges and cnidarians, and established a primitive food‐web. Such an ecological structure is recognized in modern deep‐sea coral mounds. Results from the integrated ocean drilling program (IODP) Expedition 307 for a mound in northeastern Atlantic suggested that organic carbon suspended around the density boundary in the water column is the key feature to feed the heterotrophic deep‐sea coral community. Our hypothesis is consistent with the fact that the two most primitive animal phyla (Porifera and Cnidaria) are filter feeders. The evolution of filter feeding ecosystems removed the DOC mass and may have contributed to ocean oxygenation in the terminal Neoproterozoic when animal evolution passed into the second stage, with the appearance of bilaterians.

Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation

Assemblages of clay minerals are routinely used as proxies for paleoclimatic change and paleoenvironmental conditions in Phanerozoic rocks. However, this tool is rarely applied in older sedimentary units. In this paper, the clay mineralogy of the Doushantuo Formation in South China is documented, providing constraints on depositional conditions of the Ediacaran Yangtze platform that host the earliest animal fossils in the geological record. In multiple sections from the Yangtze Gorges area, trioctahedral smectite (saponite) and its diagenetic products (mixed-layer chlorite/smectite, corrensite, and chlorite) are the dominant clays through the lower 80 m of the formation and constitute up to 30 wt% of the bulk rock. Saponite is interpreted as an in situ early diagenetic phase that formed in alkaline conditions (pH > or = 9). The absence of saponite in stratigraphically equivalent basin sections, 200-400 km to the south, indicates that alkaline conditions were localized in a nonmarine basin near the Yangtze Gorges region. This interpretation is consistent with crustal abundances of redox-sensitive trace elements in saponitic mudstones deposited under anoxic conditions, as well as a 10 per thousand difference in the carbon isotope record between Yangtze Gorges and basin sections. Our findings suggest that nonmarine environments may have been hospitable for the fauna preserved in the Yangtze Gorges, which includes the oldest examples of animal embryo fossils and acanthomorphic acritarchs.

Pb–Pb dating of black shales from the Lower Cambrian and Neoproterozoic strata, South China

Black shales occur widely in the Lower Cambrian and Neoproterozoic strata on the Yangtze Platform, South China. In this study, Lower Cambrian black shales from Xiuning section and Late Neoproterozoic black shales from Weng’an section were studied and Pb isotopic compositions were analyzed following a stepwise acid-leaching technique. The 206Pb/ 204Pb ratios in both sections show large variations, from 18.906 to 43.737 in the Weng’an section and from 24.811 to 38.110 in the Xiuning section. In contrast, the ranges for 207Pb/ 204Pb and 208Pb/ 204Pb values in both sections are relatively smaller from 15.649 to 17.126 and 37.744–38.199 in the Weng’an section, and from 16.034 to 16.783 and 38.602–39.391 in the Xiuning section, respectively. These data yielded two Pb isotope isochron ages of 536±39 and 572±36 Ma, respectively. These ages well accord with other published data and we suggest that they represent the depositional ages for the Lower Cambrian Hetang Formation and the upper Neoproterozoic Doushantuo Formation in South China.

ONTOGENY AND TAPHONOMY: AN EXPERIMENTAL TAPHONOMY STUDY OF THE DEVELOPMENT OF THE BRINE SHRIMP ARTEMIA SALINA

Abstract: Although the relationship between ontogeny and phylogeny has been of long‐standing interest to palaeontologists, the fossil record has provided little insight into the development of long extinct organisms. This has changed with the discovery of numerous assemblages of fossilized invertebrate embryos and larvae, but realising their evolutionary significance is hampered by a paucity of data on the relationship between ontogeny and taphonomy. We describe the results of an experimental taphonomy study of the development of the anostracan brine shrimp Artemia salina, which show that in conditions of aqueous aerobic and anaerobic autolysis and microbial decay, the developmental stages exhibit differential preservation potential. The most decay resistant developmental stage is the diapause cyst, encapulsating the gastrula, in which the gross morphology of the embryo can be maintained for 18 months or more in simple anaerobic conditions. Otherwise, the embryo shrinks within the cyst and cellular and tissue detail of breaks down as lipid droplets coalesce. Postembryonic excysted larvae decay more rapidly. The rate of decay is similar among all larval stages with the exception of the L4 larva, which resists cuticle failure for longer than later developmental stages. The larvae decay leading to liquefaction of the muscles and viscera, leaving an intact but empty and progressively shrunken and distorted cuticle that eventually loses structural integrity and collapses. Our experimental results provide an explanatory model for the phenomenal abundance of putative diapause stage embryos, in the absence of postembryonic stages, as seen in the Ediacaran Doushantuo Formation of South China and the incompleteness of fossilized developmental sequences of embryos and larvae more generally. It also cautions against the association of developmental stages in fossil deposits without additional evidence. Finally, the pattern of decay seen in larvae provides an explanation for the preservation style of Orsten‐type Lagerstätten where preservation of cuticular detail can be astonishingly fine, but extends internally to muscles and viscera only rarely.

Early diagenetic growth of carbonate concretions in the upper Doushantuo Formation in South China and their significance for the assessment of hydrocarbon source rock

Mineralogical and textural characteristics and organic carbon composition of the carbonate concretions from the upper Doushantuo Formation (ca. 551 Ma) in the eastern Yangtze Gorge area reveal their early diagenetic (shallow) growth in organic-rich shale. High organic carbon content (up to 10%) and abundance of framboidal pyrites in the hosting shale suggest an anoxic or euxinic depositional environment. Well-preserved cardhouse clay fabrics in the concretions suggest their formation at 0–3 m burial depth, likely associated with microbial decomposition of organic matter and anaerobic oxidation of methane. Gases through decomposition of organic matter and/or from methanogenesis created bubbles and cavities, and anaerobic methane oxidation at the sulfate reduction zone resulted in carbonate precipitation, filling in bubbles and cavities to form spherical structures of the concretions. Rock pyrolysis analyses show that the carbonate concretions have lower total organic carbon (TOC) content but higher effective carbon than those in the host rocks. This may be caused by enclosed organic matter in pores of the concretions so that organic matter was protected from further modification during deep burial and maintained high hydrocarbon generating potential even in over-matured source rock. As a microbialite sensu latu, concretions have special growth conditions and may provide important information on the microbial activities in depositional and early burial environments.

Octoradiate Spiral Organisms in the Ediacaran of South China

Abstract: Shales of the Ediacaran Doushantuo Formation of South China contain a new disc‐shaped macrofossil, Eoandromeda octobrachiata, characterized by eight spiral arms that were probably housed within an outer membrane. Although the presence of eight radiating structures entails a resemblance to cnidarian octocorals as well as to ctenophores, direct homologies with these extant groups are unlikely. Instead we bring attention to the helicospiral morphology of newly reported embryos from the Doushantuo Formation, suggesting that the long‐sought adults of the Ediacaran embryonic metazoans have finally been found.

Application of Atomic Force Microscopy on Observing the Ultra‐Fine Structure of the Neoproterozoic Microfossils from China

Phosphatic microfossils from the Doushantuo Formation, Guizhou, China, have been reported with preserved cellular structure or even sub-cellular structure in micron scale. However, more details in sub-micro scale have not been reported as having been found. The Fossil embryos from the acid residue of the phosphorite rocks of the Neoproterozoic Doushantuo Formation in south China have been studied with a scanning electron microscope (SEM) and an atomic force microscope (AFM). Some ultra-structures in sub-micro scale have been found by AFM on the surface of the fossil embryos. There are four types of structures found on the surface of the selected fossil embryos, the sizes of which vary from 30 to 645 nm in diameter under our AFM. One of the structures is composed of several big sub-units integrated with each other, and the size of the big sub-units is from 250 to 645 nm. Meanwhile, we also found an ultra-layer structure on the surface of the big sub-units, the thickness of which was about 10 nm. Thus we speculate that it could most probably be of biological origin. Therefore, AFM provides a new method for direct observation of the ultra-structure of the Doushantuo fossils in the sub-micro scale.

The diversification and extinction of Doushantuo‐Pertatataka acritarchs in South China: causes and biostratigraphic significance

AbstractThe Ediacaran Period immediately follows the last Cryogenian glaciation—the ∼635 Ma Marinoan or Nantuo glaciation, and it is also punctuated by another brief glaciation—the ∼582 Ma Gaskiers glaciation. It is possible that these glaciations may have had significant impact on Ediacaran biological evolution (e.g. the appearance or disappearance of Doushantuo‐Pertatataka acritarchs). Alternative hypotheses propose that the diversification of Doushantuo‐Pertatataka acritarchs was caused by the Acraman bolide impact or by emerging eumetazoans. To test these hypotheses, high‐resolution geochronological and biostratigraphic data are required. The Doushantuo Formation in South China, radiometrically constrained between ∼635 and ∼551 Ma, has the potential to clarify the global picture of early‐middle Ediacaran evolution. Here we present preliminary biostratigraphic data from the Doushantuo Formation in the East Yangtze Gorges area and new δ13C chemostratigraphic data from the Doushantuo Formation at Weng'an. These and previously published palaeontological data, aided by the tool of δ13C chemostratigraphy, indicate that the biostratigraphic record of the Doushantuo Formation is locally sensitive to the availability of specific taphonomic windows. In the East Yangtze Gorges area, Doushantuo‐Pertatataka acritarchs first appeared shortly after the termination of the Nantuo glaciation and gradually diversified throughout the Doushantuo Formation. At Weng'an, however, such acritarchs first appear—abruptly and in much greater diversity—in phosphorite of the upper Doushantuo Formation, immediately above a subaerial exposure surface. Thus, the biostratigraphic pattern in the East Yangtze Gorges area permits, whereas that at Weng'an apparently disallows, a causal relationship between the Nantuo glaciation and the diversification of Doushantuo‐Pertatataka acritarchs. We conclude that the biostratigraphic record is incomplete at Weng'an, where the early Ediacaran evolutionary history is not preserved. The South China data indicate that special attention has to be paid to taphonomic and palaeoenvironmental analysis before extrapolating local and regional biostratigraphic ranges to make global interpretations.It is less clear when Doushantuo‐Pertatataka acritarchs disappeared. Previous investigators have variously suggested that they disappeared before, at, or after, the Gaskiers glaciation. These hypotheses are difficult to test because of the lack of sedimentary evidence for the Gaskiers glaciation in South China and other regions (e.g. South Australia) where Doushantuo‐Pertatataka acritarchs are abundant. In Australia, Doushantuo‐Pertatataka acritarchs are thought to have experienced a sharp decline after the Egan glaciation, which may well be equivalent to the Gaskiers glaciation. If true, then Doushantuo‐Pertatataka acritarchs are largely restricted to the interval between the Nantuo and Gaskiers glaciations. This conclusion allows us to place constraints on the possible causes of the diversification and extinction of Doushantuo‐Pertatataka acritarchs and has important implications for the biostratigraphic significance of Doushantuo‐Pertatataka acritarchs. Copyright © 2006 John Wiley &amp; Sons, Ltd.

U-Pb sensitive high-resolution ion microprobe ages from the Doushantuo Formation in south China: Constraints on late Neoproterozoic glaciations

Two distinctive volcanic ash beds were found in the terminal Proterozoic Doushantuo Formation in south China. The lower ash bed, ;2.5 m above the cap carbonate at the base of the Doushantuo, yields a U-Pb zircon age of 621 6 7 Ma, providing the closest upper limit for a correlative of the Marinoan glaciation. The upper ash bed, near the Doushantuo-Dengying boundary, yields a U-Pb zircon age of 555.2 6 6.1 Ma, providing for the first time a direct age determination for a prominent negative d 13 C excursion (#25‰) above the Marinoan glacial level. This excursion, if interpreted to be of glacial origin, is much younger than the Gaskiers Formation (ca. 580 Ma) in Newfoundland, and perhaps the fifth or sixth such level in the Neoproterozoic. That interpretation, however, is not supported by the proliferation of organisms within strata encompassing the negative d 13 C excursion in south China and globally, by the lack of a ca. 555 Ma glacial record, and by the absence of stratigraphic evidence for sea-level change. The data call for alternative paleoceanographic models for the origin of Neoproterozoic d 13 C excursions not clearly related to glaciation.

NEW MULTICELLULAR ALGAL FOSSILS AND ACRITARCHS IN DOUSHANTUO CHERT NODULES (NEOPROTEROZOIC; YANGTZE GORGES, SOUTH CHINA)

The late Neoproterozoic Doushantuo Formation in South China contains several exceptionally preserved biotas in its cherts, phosphorites, and black shales (Xiao et al., 1998b, 2002; Zhang et al., 1998). The three taphonomic windows—silicification, phosphatization, and carbonaceous compression—responsible for most of the preservation of nonbiomineralizing organisms in geologic history are all open in the Doushantuo Formation. In combination, these three taphonomic windows have the potential to give us a clearer and more complete picture of late Neoproterozoic biodiversity. Comparison across the three also allows us to evaluate possible taphonomic biases and paleoecological heterogeneity. To accomplish these, however, a more thorough paleontological investigation of each taphonomic facies is necessary. Monographic studies on Doushantuo shales, cherts, and phosphorites (Steiner, 1994; Zhang et al., 1998; Xiao et al., 2002) demonstrate that macroscopic (millimeter to decimeter) fossils are typically preserved as carbonaceous compressions, while microscopic (micrometer to submillimeter) fossils are preserved as permineralized (silicified and phosphatized) forms. Published reports also indicate that, although both the silicification and phosphatization windows preserve microscopic organisms, taxonomic compositions of the silicified and phosphatized biotas appear significantly different. Doushantuo phosphorites contain a plethora of multicellular eukaryotes (algae and animals), a moderate diversity of acritarchs, and relatively few cyanobacteria filaments. In contrast, coccoidal and filamentous cyanobacteria and acritarchs are abundant and diverse in Doushantuo cherts; multicellular eukaryotes are comparatively rare in Doushantuo cherts (Table 1). View this table: Table 1 —List of prokaryotes, acritarchs, multicellular algae, and animal fossils from upper Doushantuo chert nodules in the Yangtze Gorges area and upper Doushantuo phosphorites at Weng’an and elsewhere in South China (updated from Zhang et al., 1998). Synonyms are not listed Since the monographic study of permineralized microfossils in Doushantuo cherts and phosphorites (Zhang et al., 1998), several reports of new microfossils and new localities have appeared in the literature …